How secure is private networking?

If I have a docker swarm cluster communicating over the private networking feature of Digital Ocean, is that traffic able to be seen by all of the droplets in the region? For example, can a droplet owned by another digital ocean user attempt to join my swarm cluster? In general, how "private" is the private traffic.

Also, can a droplet from one region talk to a droplet in another region if both droplets are owned by the same account?

As of July 2018, communication over DigitalOcean private networking is isolated to the resources within a single DigitalOcean account or Team. A Droplet owned by another DigitalOcean account has zero access to your account's private networking.

Beginning in June of 2018, communication over DigitalOcean Private Network IPs will be isolated within the account or team where they were created. For most users, this security enhancement requires no action. All Droplets that were provisioned with Private Networking will continue to be able...